1. Field of the Invention
The invention concerns a catadioptric projection objective with an object plane having a rectangular object field, a beam splitter, a concave mirror, an image plane, a first lens assembly, a second lens assembly, and a third lens assembly, where the first lens assembly is arranged between the object plane and the beam splitter. A preferred use of such a projection objective is the imaging of an object with light of a wavelength of ≦258 nm.
2. Description of the Prior Art
For projection exposure devices, with which patterns of photomasks or reticles, which are designated below in general as masks, are projected onto semiconductor wafers or glass substrates and exposed, optical projection systems of maximum resolution are used. In order to further reduce pattern width, which is substantially limited by resolution that is in turn limited by diffraction, preferably vacuum ultraviolet (VUV) rays with a wavelength of ≦258 nm are used for the smallest structures. A projection system for use in the deep UV region has become known from EP 0 475,020 A2.
The system shown in EP 0 475,020 A2 comprises at least a catadioptric system as well as a dioptric system. A mask to be imaged lies directly on a beam splitter, preferably a cube. A portion of light is reflected by the catadioptric system and deflected to the dioptric system by means of the beam splitter.
It is a disadvantage of the arrangement according to EP 0 475,020, that only intensity beam splitters are used on the one hand, and that, on the other hand, the object to be imaged is arranged directly on the beam splitter.
The use of intensity beam splitters has the disadvantage that only a fraction, for example, 25%, of the light passing through the mask reaches the object to be exposed, in the present case, the wafer.
A catadioptric projection system with intermediate image has become known from U.S. Pat. No. 5,636,066. Beam splitting is produced geometrically by means of a deflecting mirror. The deflecting mirror has an aperture or opening in its center and therefore serves as a diaphragm for a beam reflected from a catadioptric part at a concave mirror. The system according to U.S. Pat. No. 5,666,066 is constructed in such a way that an intermediate image is formed in a diaphragm plane of the deflecting mirror.
Another system with a geometric beam splitter, which is designed as a deflecting mirror, has become known from U.S. Pat. No. 5,691,802. A projection objective known from U.S. Pat. No. 5,691,802 also involves an intermediate image, wherein the intermediate image is formed in front of a catadioptric lens assembly in a region of a first lens assembly. It is a disadvantage in the system known from U.S. Pat. No. 5,691,802 that this arrangement necessarily involves an off-axis system due to the geometric beam splitting.
U.S. Pat. No. 4,302,079 involves a system with a polarizing optical beam splitter. A change of direction of polarization of a beam reflected by a concave mirror in a catadioptric lens assembly is provided by means of induced birefringence.
U.S. Pat. No. 4,896,952 shows a system with a polarizing optical beam splitter, wherein a change in direction of polarization is achieved in a catadioptric lens assembly by means of a λ/4 plate.
DD-C-215,179 also shows a system with a beam splitter. The beam splitter is formed as a partially transparent beam splitter cube. The system according to DD-C-215,179 has two identical mirrors at two of four sides of the beam splitter cube, which are perpendicular to a pre-given plane, as well as two dioptric subassemblies at the other two sides of the beam splitter cube. The dioptric subassemblies are designed such that a Petzval sum of the two systems extensively compensates that of the mirrors.
A catadioptric projection objective without intermediate image has become known from EP-A-0 350,955, in which a first lens group or a first lens assembly is provided between an object, i.e., a reticle, and a beam splitter; a second lens group is provided between the beam splitter and a concave mirror; and a third lens group is provided between the beam splitter and an image plane.
U.S. Pat. No. 5,808,805 and U.S. Pat. No. 5,999,333 show a catadioptric objective with an intermediate image and a beam splitter as well as at least two lens assemblies, whereby the lens assemblies are constructed in such a way that the intermediate image comes to lie in a vicinity of the beam splitter surface of the beam splitter. According to U.S. Pat. No. 5,808,805, a beam splitter, for example, a beam splitter cube, is used as the beam splitter. U.S. Pat. No. 5,999,333 shows a use of a mirror as a geometric beam splitter.
A disadvantage in the systems with geometric beam splitting is that this involves an off-axis system. Both the system known from U.S. Pat. No. 5,808,805 as well as the one known from U.S. Pat. No. 5,999,333 have a very large range of angles of incidence of back-reflecting beams from a concave mirror impinging on a surface of a beam splitter.
U.S. Pat. No. 5,861,997 shows a system similar to that of U.S. Pat. No. 5,808,805 and that of U.S. Pat. No. 5,999,333 with two intermediate images, wherein one intermediate image again comes to lie in a vicinity of a beam splitter, so that large angles of incidence occur at the beam splitter.
Due to the large range of angles of the radiation impinging on the beam splitter layer, quality of the image is reduced, since reflectivity and transmission of the beam splitter layer depend on an angle of incidence on the beam splitter layer, and different intensity distributions result with different angles of incidence.
In order to avoid this disadvantage, EP-A-0 602,923 proposes providing a lens in front of a beam splitter, by means of which radiation impinging on the beam splitter is made parallel.
A beam path that is made parallel is also realized in a catadioptric projection system known from U.S. Pat. No. 5,771,125. A disadvantage of the arrangement according to EPA-0 602,923 and that of U.S. Pat. No. 5,771,125 is that positive refractive power of a mirror in a catadioptric part is not compensated. This means that a beam that impinges on a beam splitter layer is parallel, but such is not so for a beam that is back-reflected after reflection at a concave mirror. In one direction, therefore, rays impinging onto the beam splitter layer have a large range of angles of incidence than do rays impinging from the other direction. This has the consequence that a pure splitting into polarization directions cannot be achieved. Double images and a loss of contrast are a result of this effect.
In order to minimize an angle of incidence on a beam splitter layer, DE-A-4,417,489 proposes to arrange at least one convergent lens that makes a light beam impinging onto the beam splitter layer parallel and a divergent lens group having a divergent lens after the beam splitter in a catadioptric lens assembly, in order to compensate for an effect of the convergent lens for making the beam impinging onto the beam splitter layer parallel. In addition, another convergent lens is provided on an image side after a beam splitter prism in order to compensate for an effect of the divergent lens group in a case of a beam reflecting back from a concave mirror in double passage.